1. Field of the Invention
The present invention relates to a cell pack of a proton exchange membrane fuel cell and a direct methanol fuel cell.
2. Description of the Related Art
A proton exchange membrane fuel cell (PEMFC), which is a future clean energy source which can substitute for fossil energy sources, has a high power density and a high energy conversion efficiency. In addition, a PEMFC can operate at a room temperature and can be miniaturized and hermetically sealed so that it can be applied to low emission automobiles, generating systems for home use, mobile communication equipment, medical instruments, military equipment, equipment for the space industry and so on. That is, a PEMFC can be applied to a variety of fields.
A PEMFC is a generating system which generates direct current electricity from the electrochemical reaction between hydrogen and oxygen. For the PEMFC, a single cell includes an anode, a cathode and a proton exchange membrane interposed between the anode and the cathode. The proton exchange membrane of the PEMFC has a thickness of 50-200 xcexcm and is formed of a solid polymeric electrolyte. Each of the anode and the cathode of the PEMFC is composed of a gas diffusion electrode (hereinafter, an anode or a cathode is referred to as a gas diffusion electrode) which includes a support layer for supplying fuel gas and a catalyst layer on which oxidation and reduction of the fuel gas and oxidant gas (hereinafter, fuel gas, oxidant gas and liquid fuel is referred to as fuel) proceed.
When a reactant gas is supplied to such a PEMFC, oxidation proceeds on the anode of a gas diffusion electrode so that hydrogen molecules are converted into hydrogen ions and electrons. The hydrogen ions are transferred to a cathode via a proton exchange membrane, and reduction proceeds on the cathode. In other words, oxygen molecules receive electrons so that they are converted into oxygen ions. The oxygen ions react with hydrogen ions from the anode so that they are converted into water molecules.
In the gas diffusion electrode of a PEMFC, the catalyst layer is disposed between the support layer and the proton exchange membrane. The support layer is formed of carbon cloth or carbon paper and is surface-processed so that a reactant gas, water transferred to the proton exchange membrane and water generated from reaction can easily pass through the support layer.
A direct methanol fuel cell (DMFC) has the same structure as a PEMFC. However, instead of using hydrogen gas as a reactant gas, liquid methanol is supplied to an anode, and oxidation proceeds due to action of a catalyst, generating hydrogen ions, electrons and carbon dioxide. Such a DMFC is less efficient than a PEMFC, but it can be easily applied to portable electronic equipment because fuel is injected in a liquid state.
For the above two kinds of fuel cell, a single cell generates less than 1 V, practically. Accordingly, to generate a high voltage, a plurality of single cells are superposed and electrically connected in series. To collect electricity generated, fuel flow fields and bipolar plates as collector plates as many as superposed cells are used. A fuel flow area may be realized as a metal mesh, but it is usually inscribed on a graphite block as a collector plate which is conductive, can be hermetically sealed and has a predetermined or greater thickness.
However, when such a fuel flow area is used, a flow path of a complex structure is required to consecutively supply fuel and oxygen throughout a stack of single cells starting from the outermost single cell to the innermost single cell without making the fuel and oxygen mixed. Such a fuel flow path of a complex structure has a high probability that liquid or gas which is supplied to the stack is leaked. In addition, since a plurality of collector plates should be superposed, it is difficult to hermetically seal a stack and to reduce the size and weight of the stack so that the power density is degraded. The outermost portion of the stack and the middle portion thereof have different internal resistance, temperature and humidity so that a high load can partially occur in a single cell. Consequently, the duration of the stack can be shortened.
When high power is required, it is advantageous to use such a stack method even if the method has drawbacks described above. However, when a stack is used for obtaining low power, e.g., of the applications of electronic equipment, a monopolar cell pack structure making up for the above drawbacks is advantageous.
U.S. Pat. No. 5,925,477 discloses an assembly of single cells which has a structure as shown in FIG. 1. Referring to FIG. 1, a plurality of single cells in which anodes 4, 5 and 6 and cathodes 7, 8 and 9 are superposed on the top and bottom of the membranes 3, respectively, are arranged in line. The cathodes 7 and 8 of single cells are electrically connected in series to the respective anodes 5 and 6 of adjacent single cells by conductors 2 through the overlapping of the electrode area of one single cell with the opposite electrode area of the next cell. In such a structure, a fuel flow field is provided by a graphite plate (not shown) on which a flow path is formed.
The graphite plate should be designed so that a fuel path can be provided from the outside of a cell to accomplish the fuel flow between electrodes. Moreover, in the structure in which the membrane and the upper and lower electrodes of a single cell on which electrochemical reaction proceeds should be bent, catalytic reaction is concentrated on bent portions, shortening the duration of an electrode.
To solve the above problems, it is a first object of the present invention to provide a fuel cell having a fuel flow field therewithin so that it is not necessary to separately design a fuel flow path.
It is a second object of the present invention to provide a fuel cell pack which has a simple structure and can be easily manufactured.
It is a third object of the present invention to provide a fuel cell pack having an improved power density and an improved degree of freedom in designing a single cell.
Accordingly, to achieve the above objects of the invention, in a first embodiment, there is provided a fuel cell pack including a plurality of cells each having a membrane in its middle and a cathode and an anode at both sides of the membrane, collector plates contacting the cathode and the anode, respectively, in each cell, and an electrical connection member for electrically connecting adjacent cells. Here, at least two cells are provided. The cells are evenly disposed on an arbitrary plane with a hollow interposed between two adjacent cells, and the electrical connection member is positioned in the hollow. The fuel cell pack includes a porous fuel diffusion member contacting the anode of each cell; a porous air contact member contacting the cathode of each cell; an anode end plate and a cathode end plate disposed at the side of the anodes of the cells and at the side of the cathodes of the cells, respectively, for protecting the cells; fuel supply and discharge means for supplying fuel toward the anodes in the hollow and discharging the fuel; a fuel flow stopper disposed at a portion at the part of the cathodes in the hollow, the fuel flow stopper preventing fuel flowing at a portion at the part of the anodes in the hollow from flowing toward the portion at the part of the cathodes in the hollow; and a sealing member for sealing the anodes of the cells and the portion of the hollow corresponding to the anodes.
In a second embodiment, there is provided a fuel cell pack including a plurality of cells each having a membrane in its middle and a cathode and an anode at both sides of the membrane, collector plates contacting the cathode and the anode, respectively, in each cell, and an electrical connection member for electrically connecting adjacent cells. Here, at least two cells are provided. The cells are disposed on both sides of an intermediate layer, which is provided with fuel supply and discharge means, with a hollow of predetermined volume interposed between two adjacent cells in the level direction of the intermediate layer. The electrical connection member is disposed in the hollow. The anodes of the cells disposed on both sides of the intermediate layer contact the intermediate layer. The fuel cell pack includes a porous fuel diffusion member contacting the anode of each cell; a porous air contact member contacting the cathode of each cell; first and second end plates disposed at the respective sides of the cathodes of the cells, for protecting the cells; a fuel flow stopper disposed at a portion corresponding to the cathodes of adjacent cells in a hollow, the fuel flow stopper preventing fuel flowing at a portion at the part of the anodes in the hollow from flowing toward the portion at the part of the cathodes in the hollow; and a sealing member for sealing the anodes of the cells and the portion of a hollow corresponding to the anodes.
In the first embodiment, two cells may be provided, and a fuel inlet and a fuel outlet corresponding to the hollow may be disposed on the anode end plate with a predetermined interval. Alternatively, at least two cells may be provided, and a fuel inlet corresponding to one hollow and a fuel outlet corresponding to the other hollow may be disposed in the anode end plate.
In the second embodiment, a storage space for storing fuel supplied to the anodes of the cells may be provided in the intermediate layer. Three cells may be disposed on each of both sides of the intermediate layer, and a fuel inlet and a fuel outlet which correspond to hollows, respectively, between the cells may be disposed in the intermediate layer at a predetermined interval.
In the first and second embodiments, the porous fuel diffusion member is formed of a carbon-plastic composite. Particularly, the porous fuel diffusion member may include carbon or graphite impregnated therein. The porous air contact member is formed of a carbon-plastic composite and may have a plurality of channels for the flow of air on its bottom. Preferably, the electrical connection member has a shape of a mesh.